Embrittlement-free pickling of ferrous metal



United States Patent This invention relates to limiting-the hazards ofhydro-.

gen embrittlementcand overpickling during removal of oxide scale from'ferrous met-alby acids. 7

Iron and steel products become coated with an oxide scale during theirprocess of manufacture,-the presence of which is objectionable in theirfurther processing, as for example in coating the products or formingthem by drawing in dies. A conventional method of scale removal involvescontacting the material with an aqueous solution of an inorganic acid,typically sulfuric acid, usual- 1y by immersion in a bath thereof. Sincethe acids employed are capable of dissolving the base metal, .a reaction which is more rapid than that of oxide dissolution, pitting of themetal results where acid attacks it through discontinuities in the oxidefilm. Also, after the oxide layer has been removed, continued contact ofthe metal with the acid results in undesirable dissolution of metal, acondition known as overpickling. Moreover, hydrogen produced in themetal dissolution reaction increases the likelihood of hydrogenembrittlement of the ferrous base, Which latter condition results fromabsorption of hydrogen by the base metal.

In order to alleviate the foregoing difliculties, it is common practiceto add to the pickling solution an inhibitor for the metal dissolutionreaction, thus permitting more selective action in elfecting oxideremoval and reducing hydrogen production.

It has been noted that the degree of prevention of hydrogenembrittlement during pickling, resulting with various inhibitors, isunpredictable, some inhibitors even increasing embrittlement.

The present invention concerns inhibited acid pickling solutions capableof controlling acid attack of the base metal, which solutions containinhibitors which are mark edly effective as Well in preventing hydrogenabsorption, with its resulting embrittlement elfect. These results areobtained by the addition, to the picklingacid solutions, of anacetylenic carboxylic acid, which may be acetylene dicarboxylic acid orpropiolic acid.

We have found that acetylene dicarboxylic' acid is effective when usedin the concentration range of 11 to 91 grams per liter of a 2 Nsulfuric-acid solution and that propiolic acid is elfective when used inthe concentration range of 3.5 to 28 grams per liter of a similar acidsolution.

Additional data, as to approximate useful ranges of inhibitorconcentration, are presented in Table I.

TABLE I.GRAMS INHIBITOR PER GRAM EQUIVALENT WEIGHT OF PICKLING ACIDEfiective Preferred Range, g. Rangefg.

Acetylene diearboxylic acid 4 to 70 5 to 45 Propiolic acid 1 to 28 2 toThe specimens. were pickled at 100 F. for various prese- "Ice cut andvapor degreased with trichloroethylenebefore use.

lected times up to 24'ho u rs byimmersion in either uninhibited 2 Nsulfuric-acid solutions or 2 N sulfuric-acid 5. solutions containingvarious concentrations of inhibitors. .Spec1mens were weighed prior topickling and' -aft'erjextraction' of the'hydrogen absorbed duringpickling. The hydrogen concentration in pickled steel was determined bywarmextraction at 160 C, a'ccordingtothe'method described; by R.M.Hudsonyl. K. Magor, and G. L. Stragand (-J. .An1er..Ceram. Soc. 41,23 l9 58 v TABLE IL-METAL nrssoLu'rroN AND HYD ROGEN Pro" UP IN 2 NORMALsULrURro-Aom SOLUTIONA'I) 100i Weight Loss, Hydrogen Ab- Inhibitormgjcrn. sorbed, ceJlOOg. Inhibitor 'Conc.,

4 hr. 24 hr. 4, hr. 24 hr.

None 0 21.1 130. 2 8. 3 19.1 I 5. 7 11. 4 75. 5 3.1 10.1 11.4 2.119.5 1. 5 2. 9 Acetylene dicarboxylic acid; 22.8 3.8 28.6 0.7 1.1 .45. 63.4 10. 4 0.9 0.8 91.2 2.7 13. 0 0.7 0.6 136.8 2.1 26.0 1. 7 4.2 0.7'5.3 59.9 5.6 8.6 3. 5 1.7 3. 9 1.6 1. 7 Propiolic acid 14. 0 0. 6 2. 60. 7 1. 1 28.0 3.0 6. 5 1.0 1.1 56.0 6.1 18.2 1.7 1.3

It will be appreciated that the foregoing concentration ranges andspecific examples are illustrative only, to permit ready practice of theinvention, and that such variations in concentrations and combinationsof ingredients as would occur to those familiar with the art to whichthis invention appertains are included herein and are within the scopeof the following claims.

We claim:

1. A method for embrittlement-free pickling of ferrous metal comprisingthe step of subjecting said metal to an aqueous solution consistingessentially of a strong mineral acid, adapted to pickle ferrous metal,and an acetylenic carboxylic acid selected from a group consisting ofacetylene dicarboxylic acid and propiolic acid, the concentration ofsaid latter acid being sufiicient to inhibit dissolution of the ferrousmetal and to reduce hydrogen absorption therein.

2. The method of claim 1 in which said pickling acid is sulfuric acid.

3. A method for embrittlement-free pickling of ferrous metal comprisingthe step of subjecting said metal to an aqueous solution consistingessentially of a strong mineral acid, adapted to pickle ferrous metal,and acetylene dicarboxylic acid, concentration of said latter acid beingsufficient to inhibit dissolution of the ferrous metal and to reducehydrogen absorption therein.

4. A method for embrittlement-free pickling of ferrous metal comprisingthe step of subjecting said metal to an aqueous solution consistingessentially of a strong mineral acid, adapted to pickle ferrous metal,and propiolic acid,

I the concentration of said'latte'r' acid'being sufficient to in* lingacid.

3 4 aqueous solution consisting essentially of a strong mineral2,993,864 7/1961 Monroe et a1. 252-146 X acid, adapted to pickie ferrousmetal, and propiolic acid, 3,049,496 8/1962 Monroe et a1. 252-446 saidlatter acid being'present in concentration range of 3,114,657 12/1963Stilwell 252-79.4 about 2 to 15 grams per gram equivalent Weight ofpick- FOREIGN PATENTS 468,231 3/ 1943 Italy. References Cited by theExaminer QTHER REFERENCES UNITED STATES PATENTS Raphael, AcetylenicCompounds in Organic Synthesis, 3/1944 Giloy s 252-446 X Academic PressInc., pub, New'York, 1955, pp. 81 and 82. .8/1959 Mayhew et a1.. 106-1410 7 19 1 Monroe et 1 '252 14 JOHN H. MACK, Prlmary Examiner. 7/ 1961Monroe et 252146'X R. MIHALEK, Assistant Examiner.

1. A METHOD FOR EMBRITTLEMENT-FREE PICKING OF FERROUS METAL COMPRISINGTHE STEP OF SUBJECTING SAID METAL TO AN AQUEOUS SOLUTION CONSISTINGESSENTIALLY OF A STRONG MINERAL ACID, ADAPTED TO PICKLE FERROUS METAL,AND AN ACETYLENIC CARBOXYLIC ACID SELECTED FROM A GROUP CONSISTING OFACETYLENE DICARBOXYLIC ACID AND PROPIOLIC ACID, THE CONCENTRATION OFSAID LATTER ACID BEING SUFFICEINT TO INHIBIT DISSOLUTION OF THE FERROUSMETAL AND TO REDUCE HYDROGEN ABSORPTION THEREIN.